Problems with DVD audio - If you're good with cpu's, help appreciated!

Ok, I cant get an audio playback whenever I play a DVD on my computer. I'm using an external Tascam US 122 Audio Midi Interface - the sound from my mixer is going in through there (no problems) and my sound is coming out (no problems in cubase, winamp, games, audio cds in dvd and cd drives etc).

The problem is just DVDs. There's always an audio error, and I have to reroute so the sound comes out of the computer to my speakers, rather than through the Tascam unit - but everything else works!

Anyone know what I can do? I've tried Intervideo DVD, Windows Media Player, MV2 Player and Interactual DVD viewer, and cant get sound out of any of them from my DVDs, whereas all other sound sources are fine.